Sighting devices, in particular telescopes, comprising optical assemblies for input and output coupling of transmission beams and distance measurement beams, respectively, are required for many applications, in particular geodetic, construction and military applications, for example for distance measurement and observation or for observation and imaging with a camera. The required high measurement accuracy demands maximal position and angle stability of the relevant beam paths and associated assemblies. The term “distance measurement beams” or, synonymously therewith, “measurement beams” is used for beams which are reflected in and out by means of an optical assembly in an optical instrument. They also include beams which are reflected in and out in an apparatus but are not used for the measurement, such as beams for observation in the telescope or for imaging with a camera.
For measuring distances, an integrated distance measurement module based on laser technology may be used in optical surveying instruments. For high-precision direction measurement, beam coupling of the laser beam takes place with the optical (target) axis or the optical beam path of the sighting device or general measuring device.
The input and/or output coupling of transmission beams or measurement beams is usually carried out by means of assemblies which comprise a lens, an input and/or output coupling element or optical deviation element, for example a mirror, and a frame by means of which the input and/or output coupling element or deviation element is introduced into the beam path.
In order to be able to guarantee high direction and/or distance measurement accuracy of the instruments, collinearity (parallelism) of the beam path of the sighting device, for example a telescope, and the actual laser beam coupled in must be ensured under extreme external influences (mechanical shock, vibration, temperature change) over a long period of time. The optical deviation element, for example a mirror, which couples the laser beam with the target axis, therefore must be permanently fastened stably with respect to direction and shape, without compromising the optical imaging quality of the sighting device or telescope in the surveying instrument.
The technical challenges consists in keeping a laser beam stable with respect to direction and position relative to a sighting device, in particular a telescope, for example a visual telescopic sight, the optical axis of the telescope in turn being connected stably to an angle sensor system. For the telescope with the laser beam path coupled in, an overall accuracy of seconds of arc or fractions thereof is required.
DE 196 15 601 discloses a coaxial distance measuring apparatus, in which light for a distance measurement is coupled into the optical axis of a telescope via an objective lens and an optical input coupling device, and is coupled out of the optical axis of the telescope via the same lens and an optical output coupling device. In order to introduce the input and output coupling devices into the optical beam path of the telescope, a disk-shaped transparent prism holding plate is used as a frame. The cement connection avoids the optically active face, so that the connection position should not in this case be optically problematic. The disadvantages of using such a prism holding plate as a frame for the input and/or output coupling elements, for example beam instabilities, increased overall length of the telescope or measuring apparatus, difficulty of aligning the frame in the optical beam path of the telescope, sensitivity to temperature variations and mechanical vibrations, are discussed at length in EP 1 662 278 and WO 2006/056475.
For the purpose of avoiding these disadvantages as potential error sources for a measurement and/or target observation, in order to improve the robustness of a sighting device or a telescope, as well as for reasons of cost, it is therefore desirable to minimize the number of constituent components.
For the input and output coupling of measurement beams for distance measurement on the optical axis of a telescope, in certain circumstances—in particular when the aforementioned input coupling element does not fulfill the function of light output coupling—a further optical element is required by which a part of the radiation reflected and/or scattered by an object to be surveyed is transmitted for observation in the telescope and another part is directed to a reception device of the distance meter. In the telescope described in DE 196 15 601, this splitting is carried out by means of a dichroic mirror which is introduced as a further additional element into the beam path of the telescope.
In a tacheometer telescope disclosed in EP 1 081 459 the splitting is carried out by means of a beam splitter prism having semireflective faces. In addition to this beam splitter prism, an input coupling mirror for the input coupling of measurement beams must be fastened in the tacheometer telescope by means of a frame, so that the aforementioned inaccuracies such as instabilities of the beam direction again occur. With each additional element, the risk of the influence of perturbing environmental effects, as well as the disadvantageous effect of reflection losses, increase. Another substantial disadvantage is the space requirement of each individual component and the increased overall length of the optical instrument due to this.
In order to solve the space requirement for frames of additional input or output coupling elements in the beam path or the optical axis of the objective lens, various proposals have recently been disclosed, a common feature of which is that the optical input or output coupling element or deviation element is connected to the planar surface of a planoconvex or planoconcave lens, in the region of its optical axis.
U.S. Pat. No. 6,545,749 discloses a laser distance meter which comprises an optical system having a planoconvex lens and a prism, which is fastened on the planar face of the lens. Transmission ray bundles, transmitted by a laser transmission unit, are deviated by the prism as a deviation element via the planoconvex lens in the direction of a target object. The ray bundles reflected and/or scattered by the target object are collected by the planoconvex lens as reception ray bundle and guided to a detector.
Since for a distance meter—and generally for imaging on a detector or sensor with little or no spatial resolution—no image-forming imaging qualities of the optical components used are required in comparison with a high spatially resolving imaging quality required for a telescope, the plane lens can be used without further measures to improve the imaging quality for this application. On the one hand correspondingly larger imaging errors occur owing to the planar face of the lens, so that the image circle radius—the radius of the dispersion circle in the image plane caused by imaging errors—is greater by a multiple (for example 20 times) than is possible or acceptable for an imaging system of a telescope. The diameter of the image circle (light spot of the reception ray bundle in the image plane) can for example be more than 100 μm, while it should only be from 1 to a few micrometers in a telescope. On the other hand, since conventional photodetectors without requirement for imaging spatial resolution have a photosensitive region with diameters of from 200 μm to 1000 μm, all of the reception radiation can be acquired and evaluated despite the increased image circle.
In order to permit the use of a combination, similar to that in U.S. Pat. No. 6,545,749, of a “plane lens” (planoconvex or planoconcave lens) with an optical deviation element to form an optical assembly in a telescope and to make it possible to fulfill the requirements in this regard for the imaging quality, WO 2006/056475 discusses various possibilities and proposes measures by which imaging errors, in particular due to the “plane lens” can be avoided or corrected and a required conventional visual imaging quality can be achieved.
However, the aforementioned proposals in U.S. Pat. No. 6,545,749 and WO 2006/056475 for resolving the space requirement for frames of additional input or output coupling elements in the beam path or the optical axis of the objective lens have several disadvantages and/or problems for their use in common:                A connection which is stable, in particular against mechanical vibrations, of a deviation element with a planar fastening face on a lens requires a corresponding planar face of the lens.        If the lens and the deviation element have not been manufactured integrally, for example by injection molding, such an assembly consisting of a lens and deviation means with only one common connecting surface, produced for example by adhesive bonding, is still susceptible in terms of its stability for example to thermal stresses due to temperature variations and to mechanical vibrations.        Since there is an adhesive bonding of the deviation means and lens over a relatively large area within the optical beam path of the lens, very stringent requirements must be placed on the optical properties of the adhesive. For example, the adhesive must be optically transparent and as far as possible also free from fluorescence and luminescence. Furthermore, the adhesive should have an equal or similar refractive index to the aforementioned lens.        The aforementioned stringent requirements on the optical properties of the adhesive greatly restrict the selection range for an adhesive also having mechanically optimal properties: in order for the optical deviation element, for example a mirror, which couples the laser beam to the target axis, to be fastened in a directionally stable fashion, the adhesive must have only a low thermal expansion coefficient; this is because during use, temperatures of between about −40° C. and +70° C. can occur in practice and the directional stability, required at the order of magnitude of seconds of arc, must not be compromised in such a wide temperature range. Furthermore, the water absorption of the adhesive should be as low as possible, even under conditions of up to nearly 100% relative humidity.        The adhesive bonding of ground surfaces is in principle difficult and potentially of lower stability under strongly varying environmental influences, since the adhesively bonded area is relatively small owing to low surface roughness.        The use of planoconvex or planoconcave lenses, as described in EP 1 662 278 and WO 2006/056475, entails considerable extra outlay, as likewise disclosed in these two documents, in order to achieve the end result despite a good imaging quality of a telescope.        